1. Field of the Invention
This invention relates generally to pipes for the transfer of fluids underwater and is more particularly directed to a compliant pipe for the delivery of cold water from the bottom of a sea to the warmer surface thereof in order to utilize the differences in temperature in converting energy from the thermal form to another, more useful form.
2. Description of the Prior Art
Proposed systems for ocean thermal energy conversion utilize the energy which the sun has stored in heating the upper layers of the ocean. In the conversion of this thermal energy to useful work, prodigious quantities of both warm and cold water are required. The warm water evaporates the working fluid (e.g. ammonia) and the cold water condenses it again. Water sufficiently cold is found deep in the ocean, in the main thermocline or even below. In one class of ocean thermal energy conversion power plant, cold water from depths in the approximate range of 1500 to 3000 feet is pumped upward to the conversion machinery on the surface, or just below the surface.
To conduct this cold water, pipes in the approximate dimensional range of 60 to 120 feet in diameter are under conceptual development. The following materials have been investigated or proposed: steel, aluminium, titanium, concrete and fiberglass. Most generally, the pipes are to be assembled on the plant site from prefabricated cylindrical sections. On one design, however, the large pipe is to be assembled in the fashion of a circular stockade of oil well drilling pipes. Another concept employs an envelope of rubberized fabric around a skeletal frame. To provide both insulation and buoyancy, one design employs air voids in the pipe walls. All cold water pipe designs so far reported may be characterized as "rigid".
The primary problems with these cold water pipe concepts may be summarized by the words: "too heavy, too awkward". For example, the weight associated with a concrete pipe, 60 feet in diameter and 1500 feet in length, reflects in a massive buoyancy requirement in the near surface machinery unit which must support the pipe. High stress levels in the pipe walls, particularly at joints, due to both static weight and dynamic forces present difficult design problems. To maintain these stress levels within acceptable bounds, pipe sections must be made generously thick, which in turn mitigates against weight reduction and adds to the weight lifting requirements of the near surface machinery unit. Steel reinforcement of the concrete pipe walls is fraught with corrosion problems which are exacerbated by the high ambient pressures of the pipe's environment. The rigging and handling problems associated with assembly and deployment at sea of such a massive structure are truly monumental. Largely because of these problems, retrieval or replacement of the coldwater pipe at sea is not contemplated except in the most unusual circumstances. This leads to the imposition of extremely rigid reliability specifications on the cold water pipe.
The present invention contemplates avoiding the above indicated objections to the use of rigid pipes by providing a compliant or pliable pipe of low longitudinal and dynamic stresses.
In operation compliant underwater pipe relies on internal pressurization which inflates the pipe and allows it to conduct the fluid. A coldwater pipe constructed of compliant underwater pipe solves all of the problems noted above. It is light in weight; some configurations of compliant underwater pipe may be adjusted to zero weight in water (neutral buoyancy). The light (or zero) weight results in low static stress, which in turn results in long life. Light weight and elasticity insure that dynamic stresses are kept low. It is convenient to store, deploy and retrieve; this leads to efficient and rapid deployment and retrieval. A casualty in the coldwater pipe or pump module which would cause a major, prolonged shutdown of an ocean thermal energy conversion plant using a rigid cold water pipe, would result only in a minor inconvenience, with a short period off-line, for one equipped with compliant underwater pipe. Corrosion vanishes as a problem from the non-metallic compliant underwater pipe. Also, low cold water pipe weight permits the use of smaller surface machinery units thereby resulting in smaller costs of installation and operation.